Lebanon believes Saad al-Hariri, who resigned as prime minister on Saturday while in Saudi Arabia, is being held by Riyadh, and Beirut plans to work with foreign states to secure his return, a top Lebanese government official said.

Hariri's shock resignation read from Saudi Arabia pitched Lebanon into a deep political crisis and pushed the country back to the forefront of a regional struggle between Saudi Arabia and Iran.

It has fuelled speculation in Lebanon that the Sunni Muslim politician, long an ally of Riyadh, was coerced into stepping down by the Saudis.

"When he went (to Saudi Arabia) he was asked to stay there and ordered to resign. They ordered him to read his resignation statement and he has been held under house arrest since," said the senior Lebanese politician, who is close to Hariri.

Saudi Arabia and Hariri aides have denied reports that he is under house arrest.

But he has put out no statements himself denying his movements are being restricted.

He made a one-day flying visit to the United Arab Emirates earlier this week before returning to Saudi Arabia.